0D Cesium Lanthanide Chlorides: Spectroscopic Insights and Optical Properties for Emerging Optoelectronic Applications

Trivalent lanthanide (Ln³⁺) ions are used in numerous applications such as light-emitting diodes, optical lasers, night vision devices, and thermal imaging. It is well known that Ln³⁺ ions have low absorption and emission intensities. Therefore, these ions are incorporated into host materials to improve their luminescent properties. In particular, halide perovskites and their derivatives are good host materials due to their high absorption coefficient, high defect tolerance, and solution processability. Each Ln³⁺ ion has unique optical and electronic properties, with emissions spanning the ultraviolet, visible, or infrared spectrum. Direct excitation of Ln³⁺ ions is difficult because their f–f transitions are parity-forbidden, resulting in very weak absorption. To overcome these limitations, dopant ions or ligands are used as sensitizers that absorb energy through allowed transitions and then transfer it indirectly to the Ln³⁺ ions, enhancing their emission intensity. This review discusses the origins of the sharp emission lines in Ln³⁺ ions, provides an overview of their electronic transitions, explores various synthesis methods, and concludes with a discussion of their potential in advanced optical and optoelectronic devices.